Major modification to your lathe. I applaud your lathe improvement.
I have a 1976 Jet 10x24 lathe. Very similar head stock construction. I found when I had pulled the spindle many years ago (because of an oil leak), that the oil level sight glass, had a single hole going straight back to the area between the taper bearing cup, and the seal (annulus). If the level dropped below this hole, the sight glass indicated there was oil, but the bearing was dry. I modified my head stock by hand drilling a 1/8" hole from the bottom of the oil sight glass reservoir to the bottom of this bearing cup and seal annulus space (both front and rear bearings). I do see now when it runs that the bearing pulls oil in from the reservoir, and shows a low level, but returns when the spindle is stopped.
I too have installed a 3phase motor on my lathe, and a VFD, but still use the belts. I did change the standard v-belts with the segmented type, which greatly reduced vibration and noise.
This lathe was originally on a wood bench, and then a sheet metal stand. I could never do parting operations as it would go into a harmonic oscillation of the headstock and ways. I purchased a 2 inch slab of steel to bolt the lathe onto. I put that slab on my Wells-Index vertical mill, and faced the hot rolled slag off. Then drill and tapped the 6 mounting holes that mate with the mounting ears on the lathe way casting. This fixed the rigidity of the lathe to where I have no more issues with cutoff operations.